A great little book about all the different kinds of castles that you might want to have in your world. It's a good mix of text and tables. It includes useful information about construction materials and what that might mean to both physical and magickal attackers, and it covers both castles based on the real-world as well as purely fictional principles (for instance, what might a demon lord use to construct a castle?).
A worthwhile read, and not a bad book to have handy at the gaming table for when your players stumble upon a castle that you hadn't planned on.